1. Field of the Invention
This invention relates generally to apparatus, such as a video tape recorder (VTR) of the so-called "helical-scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape and, more particularly, is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks for reproducing the video or other information signals recorded therein.
2. Description of the Prior Art
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During the recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. If the speed and direction of advancement of the tape are the same during a reproducing operation, as during a recording operation, then the scanning path of the head will be parallel to each record track and a servo system may control either the speed of tape advancement or the rotary speed of the head for obtaining proper scanning of the head along each track. However, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not coincide with a record track on the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be correctly or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, for example, as disclosed in U.S. Pat. Nos. 4,163,994, 4,172,265 and 4,237,399, each having a common assignee herewith, the head is mounted on the rotary drum portion by way of an electro-mechanical deflecting means, such as, a bi-morph leaf, for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks, and, in the reproducing mode of the VTR, any deviation or tracking error of the head scanning path relative to a recorded track is detected and an electrical drive signal for the bi-morph leaf is correspondingly controlled to correct the tracking error with a view to obtaining a reproduced picture of high quality.
In the non-normal or speed-varied reproducing modes, that is, when the speed and/or direction of advancement of the magnetic tape are not the same during reproducing as during recording, either a track is scanned a plurality of times in a so-called overlapped trace for a slow-motion or still motion reproducing mode, or the tracks are only scanned at intervals in a so-called skip or interval tracing for a quick or fast motion reproducing mode. In the course of each of the foregoing non-normal reproducing modes, it is necessary that, at certain times, the head be made to jump or fly-back with a pitch corresponding to an integer or whole multiple of the track pitch from the terminal end of a track which has been scanned to the initial end of the track which is to be next scanned.
In published Japanese Patent Application No. 117,106/1977 (Applicant: Ampex Corporation), corresponding to U.S. application Ser. No. 677,815, and U.K. patent application No. 1,579,854, it is disclosed to control the head jump by detecting the drive voltage to the bi-morph leaf as an indication of the deflection of the head required for accurately scanning a record track thereby, and, when the detected voltage reaches a predetermined value at which the head deflection approaches the physical limit of that possible with the bi-morph leaf or other transducer deflecting device supporting the head, the drive voltage for the bi-morph leaf is suitably controlled to cause the head to jump or skip as required. However, in the foregoing arrangement, when the head follows a recorded track which is curved or has an offset initial end, the head jump control is adversely affected, and optimum control of the head jump is not achieved. Further, the tracing form or pattern of head jumps that are effected will be varied undesirably when the bi-morph leaf or other transducer deflecting device becomes saturated or when its sensitivity, that is, ratio of its deflection to a unit change in the drive voltage, varies with time.
It has also been proposed, for example, in published Japanese Patent Application No. 113,308/79, corresponding to U.S. Pat. No. 4,296,443, to control the head jump or fly-back and track jump or change on the basis of detected variations of the phase of reproduced vertical synchronizing signals relative to reference synchronizing signals. Although this arrangement avoids influence on the tracing form of curvatures or offsets in the recorded tracks, control of the tracing form is disrupted or altered in the event of any dropouts in the reproduced video signals, and hence in the reproduced vertical synchronizing signals.
Further, the above described existing arrangements for controlling the tracing form are prone to establish undesirable tracing modes, such as, the frame still mode, in which two adjacent tracks having recorded therein the two fields of a frame are scanned alternately when providing a reproduced still-motion picture, or an unbalanced or non-uniform slow reproducing speed. Such undesirable tracking modes are particularly likely to appear when the reproducing tape speed is nearly, but not quite an integer or whole multiple of the recording or normal tape speed.